1. Field of the Invention
The present invention relates to particulates, particularly ultrafine particulates of highly white zinc oxide, more particularly the one having a tetrapod- or needle-shaped particle configuration. Furthermore, the present invention relates to particulates, particularly ultrafine particulates of highly white zinc oxide obtained by flowing zinc vapor along with a carrier gas composed of an inert gas into a reactor, simultaneously flowing an oxidizing gas containing oxygen and steam into the reactor to oxidize the zinc vapor, and quickly cooling it and to a production process thereof. Here, although ultrafine particulates in general is not strictly defined with respect to the primary particle size, fine particles having a primary particle size of about 0.1 xcexcm or less is usually called ultrafine particles (particulates).
2. Description of the Related Art
Zinc oxide has been used as a medicine, a vulcanization accelerator for rubbers, a catalyst, a material for electronic components, a paint, a pigment, a cosmetic, a reinforcing material for plastics or rubbers, etc. Generally, the production process for zinc oxide includes a liquid phase process and a vapor phase process. The liquid phase process produces zinc oxide by synthesize and precipitate zinc formate, zinc oxide, basic zinc carbonate or the like in a liquid phase, filtering and washing the precipitates, and thermally decomposing them. The liquid phase process can readily give fine primary particles. However, since it includes the steps of liquid phase synthesis, filtering, drying, etc., the zinc oxide obtained thereby must inevitably be aggregates. In practical use, zinc oxide must be slurried together with liquids as in the case of paints, pigments, cosmetics, etc., incorporated in or compounded with other solids as in the case of reinforcing materials for rubbers and plastics, or mixed with other powders as in the case of materials for electronic components. In these cases, the existence of aggregated particles considerably deteriorates the uniformity in composition or dispersibility. It is well known in the art that finer particles have more increased dispersibility. Furthermore, use of ultrafine particles can impart additional properties such as increased ultraviolet-shielding-ability while maintaining the transparency of the material containing them as in the case where they are used in cosmetics. Therefore, pulverization treatment must be performed in order to break the aggregates or obtain fine particles. However, this causes disadvantages upon use in that contamination from the vessel, pulverizing balls, etc. occurs and that the particle size distribution becomes broader. In addition, there is a limitation in making the particles finer so that ultrafine particles cannot be obtained. That is, it is very difficult to obtain ultrafine particulates of highly white zinc oxide by a liquid process.
On the other hand, there is a vapor process as a process for producing zinc oxide powders. The vapor process is to obtain zinc oxide by oxidizing zinc vapor as is well known in so-called French process and U.S. process (See Bull. Chem. Soc. Japan, 837-842, 1984). The U.S. process is a process which includes oxidization of zinc vapor containing a reducing gas and may happen to produce needle-shaped zinc oxide. However, this process has a problem in purity since lead, cadmium, etc. tends to be contained in the product. Furthermore, conglomerate particles tend to be mixed in the product. On the other hand, the French process is a process which produces zinc oxide by oxidizing zinc vapor. It gives zinc oxide in a higher purity than the U.S. process but the zinc oxide is comprised by conglomerate particles and it is difficult to obtain products made simply of needle-shaped particles.
As a method for obtaining tetrapod- or needle-shaped zinc oxide, JP-A-3150299 discloses a production process of zinc oxide whiskers by setting the temperature of the atmosphere in a heat-resistant vessel to a predetermined temperature and slowly cooling the obtained zinc vapor to promote the growth of needle-like crystals. However, the zinc oxide obtained by the process is comprised by relatively large tetrapod-shaped particles having a particle diameter of about 5 xcexcm to about 80 xcexcm. There is another problem that the above process is a production process of a batch-type so that it is unsuitable for mass production. Furthermore, JP-A-4-349118 discloses a method for obtaining needle-shaped zinc oxide by adding zinc vapor stepwise along its flow path. However, the particles obtained by the process have a particle size as large as 1 xcexcm to 20 xcexcm.
The method for obtaining fine tetrapod-shaped zinc oxide includes a process for obtaining needle-shaped zinc oxide by oxidizing and combusting zinc vapor and cooling the product at a rate of 480xc2x0 C./sec or more as disclosed in JP-B-6-05529. Even in this case, however, fine particles at best have an average particle size of about 0.2 xcexcm to about 1 xcexcm. To make them finer, it could be considered to increase the cooling rate to an extremely high rate. However, this measure results in insufficient oxidation of zinc vapor. As a result, unreacted zinc contaminates the product to color it gray so that it is very difficult to obtain ultrafine particulates of highly white zinc oxide. Furthermore, JP-A-7-118133 discloses the production method for ultrafine particulates of zinc oxide having a BET specific surface area of 70 m2/g to 110 m2/g. Such extremely ultrafine particles when in use for cosmetics will have decreased ultraviolet-shielding-ability in spite of an increase in the transparency of zinc oxide.
An object of the present invention is to provide fine particles, especially ultrafine particulates of highly white zinc oxide useful as an additive to cosmetics, pigments, paints, etc., particularly the one having a tetrapod- or needle-shaped particle configuration.
Another object of the present invention is to provide a process of producing fine or ultrafine particulates of highly white zinc oxide in a high purity and in a high yield.
Particularly, an object of the present invention is to provide fine or ultrafine particulates of highly white zinc oxide and production process thereof.
With view to obviating the above problems, the present inventors have made extensive investigations. As a result, they have found that fine or ultrafine particulates of highly white zinc oxide can be obtained that has a specified particle diameter within which the ultraviolet-shielding-ability is high, i.e., a reduced average particle diameter of about 15 nm to about 55 nm as measured by the BET-one point method (JIS R1626) and that has an L value of about 90 or more as measured by a Hunter-type color-difference meter. Also, they have found that for this purpose, the following two conditions must be met simultaneously: (1) zinc vapor is completely oxidized in a short time and (2) the reaction is terminated immediately after the completion of oxidization in order to determine the end point of the oxidation reaction of zinc vapor and avoid the collision and fusion of generated zinc oxide particles. The present invention has been achieved based on the above discoveries.
That is, the present invention relates to fine or ultrafine particulates of highly white zinc oxide having a reduced average particle diameter of about 15 nm to about 55 nm as measured by the BET-one point method and that has an L value of about 90 or more as measured by a Hunter-type color-difference meter, particularly the one having a tetrapod- or needle-shaped particle configuration and to a process for producing ultrafine particulates of highly white zinc oxide characterized by quick and complete oxidation of zinc oxide and inhibition of the growth and aggregation of particles.
According to the present invention, it has been found that to obtain ultrafine particulates of zinc oxide having an L value of about 90 or more as measured by a Hunter-type color-difference meter, it is important to oxidize zinc vapor with an oxidizing gas quickly and completely. To this end, it is found to be very effective to vaporize zinc in a vaporizer and superheat the resulting gas by a heater provided downstream the vaporizer and it is also found to be effective to use a dense silicon carbide ceramic as a material of the heater. After the vaporization of zinc, introduction of the raw material gas into the heater made of a silicon carbide ceramic enables the temperature of the raw material gas upon its discharge into the reactor to efficiently reach such a high temperature as has been never reached conventionally, i.e., as high as about 950xc2x0 C. or more. Also, a heater made of the same material can be used for the oxidizing gas to sufficiently heat it so that a temperature of about 900xc2x0 C. or more can be reached at a nozzle for discharging the gas into the reactor. In addition, it has been found that by selecting the composition of the oxidizing gas in a specified range, the raw material gas as heated as described above can be oxidized very uniformly and quickly. The new knowledge on the raw material gas and oxidizing gas has led to very quick and complete oxidation of zinc vapor and as a result, it has been found that ultrafine particulates of highly white zinc oxide can thus be obtained. By enabling such a quick oxidation of zinc vapor, no coloration of product occurs in spite of a decreased length of the reaction part and it has been successful in obtaining ultrafine particulates of highly white zinc oxide having a reduced average particle diameter of about 15 nm to about 55 nm as measured by the BET-one point method and that has an L value of about 90 or more as measured by a Hunter-type color-difference meter, particularly the one having a tetrapod- or needle-shaped particle configuration.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments with reference to the attached drawings.